This invention relates to an electrical current sensor, in particular for the measurement of a time-varying electrical current in a conductor.
A well-known means of measuring an electrical current flowing through a primary conductor is by measuring the generated magnetic field with a magnetic field sensor, positioned around or in the proximity of the primary conductor. Time-varying electrical currents are commonly measured by means of a coil, known as a Rogowski coil, bent around the primary conductor. The varying magnetic field induces a voltage between ends of the coil conductor proportional to the rate of change of the current (di/dt) in the primary conductor. The output signal of the Rogowski coil in conventional designs is thus integrated to obtain the primary current reading. There are different known ways of providing Rogowski coils, which may for example be in the form of wound air coils, or coils formed by conductive traces on a circuit board. Examples of prior art Rogowski coils are described in Murgatroyd, P. N., Making a Rogowski coil, Meas. Sci. Technolog. 2 (1991), pages 1218-1219; or Ray, W. F., Murray, K. D., The use of Rogowski coils for current wave-form measurement in power electronic circuits, EPE 1991, pages 3379-3383; or Heumann, K., Magnetischer Spannungsmesser hoher Prxc3xa4zision, Elektrotechnische Zeitschrift, A83 (1962) 11, pages 349-356; or U.S. Pat. No. 5,442,280, or U.S. Pat. No. 5,414,400.
Conventional Rogowski coil current sensors typically have coils with a large number of windings, for example the Rogowski coil described in U.S. Pat. No. 5,414,400 is provided with about 250 windings, in order to increase the sensitivity to magnetic fields and reduce the effects of an out-of-center primary conductor. Such sensors function reasonably well within a small and defined frequency band, and positioned at relatively large distance from other conductors that generate magnetic interference fields. Furthermore, such known Rogowski coil sensors are limited to the measurement of time-varying electrical currents having frequencies up to orders of magnitude of 103-106 Hz. These characteristics of conventional current sensors have been, until the present invention, accepted as technical limitations of current measurement with Rogowski coils.
Certain devices, in particular electrical power converters and inverters, may produce in the time domain large discontinuities in the current slope resulting from the switching procedures which cause frequencies in the frequency domain up to approximately 108 Hz. Conventional current sensors of the Rogowski coil-type are not able to accurately and reliably measure such rapid rates of change of current. Furthermore, conventional current sensors of the aforementioned type are sensitive to magnetic interference fields which are often very difficult to avoid in view of the proximity of conductors in existing electrical power switch-mode devices, where the measurement of current is often not a primary consideration during design of the device.
It is an object of the present invention to provide a versatile yet precise sensor for measuring time-varying electrical currents flowing in a conductor.
It is particularly advantageous to provide a sensor for measurement over a large frequency range, in particular a frequency range extending into the range 106-108 Hertz.
It is also particularly advantageous to provide a current sensor with reduced sensitivity to magnetic interference fields.
It is also advantageous to provide a current sensor that is compact, cost effective and that can be implemented without interrupting the primary conductor.
It is also advantageous to provide a cost effective current sensor with a high sensitivity to the field to be measured and a high signal to noise ratio.
Objects of this invention have been achieved by providing the sensor described below.
Disclosed herein is a current sensor for measuring time-varying electrical currents in a portion of a primary conductor, the sensor including a sensor conductor comprising first and second ends for connection to a signal processing circuit, a first conductor portion extending from the first end and having a plurality of windings to form a first coil portion extending from a first coil portion extremity to a second coil portion extremity, and a second conductor portion returning from the second coil portion extremity to the first coil portion extremity and to the second end, centres of the windings defining a median coil plane substantially parallel to the magnetic field generated by the current to be measured, the sensor conductor defining one or a plurality of interference field surfaces projected on the median plane enclosed by clockwise or anti-clockwise interference field current circulating portions of the sensor conductor, wherein the second conductor portion is arranged such that the surface area of the one or more projected interference field surfaces is close to zero or significantly smaller than the cross-sectional area of the windings times the number of windings, and/or such that the voltage induced by a magnetic field component orthogonal to the median plane in the clockwise interference field circulating portions is substantially cancelled by the voltage induced in the anti-clockwise interference field circulating portions.
Advantageously therefore, the effect of magnetic interference fields, in particular from external conductors in the proximity of the sensor, is cancelled or reduced to a minimum by reducing the area enclosed by the sensor conductor subject to the effects of magnetic interference fields, in particular the magnetic field component orthogonal to the median plane.